This invention is related to novel ultraviolet (UV) radiation-absorbing organo-silicone ester and ether molecules, in both monomeric and polymeric forms. Preferred methods for their production from silicone hydrides and organic UV absorbers and usage in coatings prepared via acid catalyzed alkoxysilane sol-gel synthesis on various glass (soda-lime, etc.) substrates are provided. Coatings on glass substrates, incorporating embodiments of the disclosed invention, aid in preventing UV radiation from degrading underlying articles.
Numerous organic and inorganic compounds that absorb UV radiation have been described in the prior art. As used herein and in the accompanying claim, a UV absorber is defined to mean a compound that absorbs radiation of wavelengths starting in the near visible portion of the electromagnetic spectrum, around 400 nanometers to the so-called vacuum ultraviolet, around 200 nanometers in wavelength. These compounds are often used to protect items underlying transparent, semitransparent and/or translucent substrates from the degradative effects of ultraviolet radiation absorption. Applications include, but are not limited to eyeglasses, windows, and glass panes in paintings, art prints, and the like. UV absorbers coated onto glass for the protection of paintings, pictures, and the like are of particular importance with a view to the present invention.
One class of polymers useful as matrices for UV absorbers are crosslinked polysiloxanes synthesized by sol-gel methods. As used herein and in the accompanying claims, reference to a xe2x80x9csol-gelxe2x80x9d synthesis is intended to mean any method of preparation of a poly- or oligo-siloxane using hydrolysis of alkoxysilanes via acid, base, or other useful catalysts, e.g. coordination catalysts, to produce oligomer and/or polymers useful for preparing films and coatings. Polysiloxanes produced via sol-gel synthesis and applied in thin coatings are generally transparent to electromagnetic radiation in the so-called xe2x80x9cvisiblexe2x80x9d wavelengths of the electromagnetic spectrum where the human eye absorbs such radiation.
However, it is generally recognized in the sol-gel reaction art that effectively dispersing and retaining organic-base additives such as radiation absorbers, is problematic. Organic UV absorbers are often insoluble in silane/silicone monomers and polymers and can require extensive mixing procedures to produce a uniform, albeit unstable, dispersion useful for various applications. Additionally, with a view to the prior art, monomeric UV absorbers are known to evaporate during heat processing which is required to finally cure siloxane systems. Further, monomeric UV absorbers often exhibit a yellow hue or color when they are not fully dispersed. This color effect can require correction by adding expensive dyes which further complicates the formulation and cost of a UV absorbing coating system. Finally, monomeric UV absorbers tend to both 1.) leach out of polysiloxane matrixes over time and 2.) deactivate via oxidative decomposition.
One potentially useful method of retaining UV absorbers is to covalently bond them onto alkoxysilane monomers for subsequent incorporation into a polysiloxane matrix via sol-gel synthesis. Several chemistries have been utilized in this regard. One modified alkoxysilane useful in the preparation of certain classes of coating solutions is disclosed in U.S. Pat. No. 4,051,161 to Proskow, the entire disclosure of which is hereby incorporated by reference. Proskow discloses several alkoxysilane-benzophenone monomers prepared by reacting an alkoxysilane that contains an epoxide moiety with a hydroxyl group on a hydroxybenzophenone to form an xcex1, xcex2 hydroxy-ether which covalently links the hydroxybenzophenone and the silane. Proskow""s invention appears to be limited to monomeric species for incorporation into polymers containing polysilicic acid (e.g. silica) and hydroxylated fluorocopolymer systems. However, there are problems with and disadvantages to directly bonding an alkoxysilane modified UV absorber (e.g. a hydroxybenzophenone) into a sol-gel polymerized matrix as disclosed in Proskow. Without wishing to be bound by a particular theory, it is believed that, when monomers having multiple sets of multiply reactive groups (e.g. multiple trialkoxysilanes groups) are polymerized into a film-forming network, stresses can be generated throughout the forming film. These stresses can cause localized failures in coatings thus leading to premature film oxidation and loss of UV absorbers.
Finally, because of their low viscosity, sol-gel polymerizations often require the addition of so-called xe2x80x9clow control agentsxe2x80x9d to provide an increase in viscosity. These are typically higher molecular weight oligomers that are used for viscosity control during the monomer reaction and curing stages of film preparation. However, it would be desirable to eliminate these materials as they can cause potential negative effects in the ultimate physical properties of cured siloxane films, e.g. hardness.
It is apparent from the foregoing discussion that improvements in the art of siloxane/silane sol-gel polymerizations related to the addition of functional additives are desirable. Those desirable improvements include, but are not limited to: (a.) the ability to uniformly incorporate organic-base UV absorbers into a silane matrix without reliance on variable mixing schemes and complicated preparation procedures, (b.) the elimination of additional colorants, (c.) minimizing or eliminating the addition of flow control agents, (d.) minimizing possible mechanical stresses in cured films, and (e.) minimizing the loss of additives, such as UV absorbers, caused by processing heat and environmental degradation. It is a purpose of the present invention to fulfill any and/or all of these needs as will be apparent from the following detailed disclosure of this invention.
It is therefore one objective of the present invention to provide UV absorbers which can be uniformly incorporated into polysiloxanes, prepared by sol-gel reactions, to produce films and coatings having uniform radiation absorption characteristics.
It is another objective of the present invention to provide UV screening monomers, macromers and polymers that resist volatilization and decomposition in curable compositions of alkoxysilane monomers.
It is an additional objective of the present invention to provide UV screening monomers, macromers, and polymers which will exhibit low color in polysiloxane films curable on transparent, semitransparent, or translucent substrates.
It is an additional objective of the invention to provide UV absorbing monomers, oligomers, and polymers that display behavior as flow control agents.
It is an additional objective of the invention to provide benzophenoxy-alkoxysilane monomers that are capable of hydrolyzing into polymerizible silanols and unbound hydroxy-benzophenones when utilized in a silane based sol-gel polymerization.
An additional objective of the invention is to provide alkoxysilane based film-forming solutions that are curable onto a substrate and will yield substantially insoluble polysiloxane films having uniform UV absorption characteristics.
It a further objective of the invention to provide methods of preparing and using the disclosed molecules, macromers, polymers and coating compositions.
These and other objectives are exemplified by certain non-limiting embodiments of the invention as disclosed in the following examples and descriptions.